The present invention relates to high-efficiency coding systems that use a smaller amount of coding to efficiently code signals in recording, transmission and display apparatus that perform the processing of digital signals. More particularly, it relates to a coded output data amount control system that makes uniform the amount of data which is coded and output.
One high-efficiency coding system that is conventionally known uses variable length coding so that the data amount per sampling is made smaller. In this high-efficiency coding system, prediction error signals obtained by predictive coding or orthogonal transform coefficient signals obtained by orthogonal transform have a significant amount of discrepancy in the level distribution of these signals.
When coding is performed using codes that have different lengths (variable length codes such as Huffman codes) in accordance with the frequency of generation of the signal level, the average value for the coding length becomes shorter when compared to the case where fixed length codes are used.
When the variable length codes are used, it is possible to greatly reduce the data amount for the larger the discrepancies in the generation frequency of data. In addition, when compared to the case where the fixed length codes are used, the average data amount by the variable length codes can be made smaller while maintaining the same quality for reproduced signals.
However, the data amount by the variable length codes is not constant and so when it is desired that a data amount for each predetermined certain division be constant, it is necessary to control the amount of generated data so as to make it constant.
One general system of data amount control is feedback control by a data output buffer. This reduces the number of quantization steps and makes the data amount smaller when the amount of coded data has increased and the amount of data that is stored in the buffer has become large. Control systems such as this are used in systems for television conference and the like that perform continuous signal processing.
However, in case of the recording and reproduction of video signals to and from a storage and recording medium such as a recording disk or recording tape or the like, it is necessary to have a reproduction function in special reproduction modes (such as slow motion reproduction, high-speed reproduction and the like) in addition to the normal reproduction mode. In this case, image data that is to be made the object should desirably be divided into divisions of single units such as for each one-frame cycle, and such that the data amount is constant. When there is the editing of image data that has been coded, replacing image data requires that the data length be fixed in unit of each field cycle to several frame cycles.
In cases such as these, this type of feedback control maintains the average value for the data amount at a constant value and so the data amount cannot be accurately stored within a constant value of a desired unit.
The same inventor as of the present invention has already proposed in Japanese Patent Application No. 62887-1990 applied for on Mar. 14, 1990, a data amount control system that makes the data amount to within a constant value.
With the system disclosed in this Japanese Patent Application, the activity of the orthogonal transform coefficients obtained by orthogonal transform of incoming signals is first obtained. This activity is a quantity that expresses the degree of change of an image (and is small for when there is a small portion of the image which is the same, and large for when there is a large portion of the image which is complex), and is determined by the mean square of the alternating coefficients that express the spatial frequency components of luminance of pixels inside one block obtained by orthogonal transform (where one block is a group of a predetermined number of pixels of an original image).
This activity is used as the basis for determining a temporary quantization step so that the data amount inside one frame becomes constant. This is feed forward control.
Furthermore, this activity is used as the basis for determining the predictive data amount for each block. The predictive data amount is subtracted from the data amount for each actual block obtained by quantization of the orthogonal transform coefficients and variable length coding thereof thus quantized, and this result is then accumulated. The accumulated result is used as the basis to change the quantization step. By adding this changed quantization step and the temporary quantization step, a final quantization step is obtained which is used for quantization of the orthogonal transform coefficients. This is feedback control. Moreover, with this system, the orthogonal transform coefficients are stored once in a coefficient memory for the purpose of compensation of delay by feed forward control, and these stored orthogonal transform coefficients are quantized.
Because of feed forward control, it is necessary to have a memory for the coefficients or a frame memory of from one to several frame portions which are the units for which the data amount is to be made a constant value.
When the feed forward and feedback controls described above are used for interframe coding having independent frames at constant intervals, such as disclosed in U.S. Pat. No. 4,985,768 issued on Jan. 15, 1991, by the same inventor of this invention, an output data amount of an interval of the independent frames is desirably constant. Memories for four to six frames are thus required for compensation of a delay due to feed forward control. In such coding in which an output data amount is controlled, an ideal data amount of each frame changes largely with the degree of motion of an image and the quality of the image deteriorates if the amount of data for one frame is fixed.